Gas separation is important in many industries for removing undesirable contaminants from a gas stream and for achieving a desired gas composition. For example, natural gas from many gas fields can contain significant levels of H2O, SO2, H2S, CO2, N2, mercaptans, and/or heavy hydrocarbons that have to be removed to various degrees before the gas can be transported to market. It is preferred that as much of the acid gases H2S and CO2 be removed from natural gas as possible to leave methane as the recovered component. Small increases in recovery of methane can result in significant improvements in process economics and also serve to prevent unwanted resource loss. Ti is desirable to recover more than 80 vol %, particularly more than 90 vol %, of the methane when detrimental impurities are removed.
Additionally, synthesis gas (syngas) typically requires removal and separation of various components before it can be used in fuel, chemical and power applications because all of these applications have a specification of the exact composition of the syngas required for the process. As produced, syngas can contain at least CO and H2. Other molecular components in syngas can be CH4, CO2, H2S, H2O, N2, and combinations thereof. Minority (or trace) components in the gas can include hydrocarbons, NO3, NOx, and the like, and combinations thereof. In almost all applications, most of the H2S should typically be removed from the syngas before it can be used, and, in many applications, it can be desirable to remove much of the CO2.
Adsorptive gas separation techniques are common in various industries using solid sorbent materials such as activated charcoal or a porous solid oxide such as alumina, silica-alumina, silica, or a crystalline zeolite. Adsorptive separation may be achieved by equilibrium or kinetic mechanisms. A large majority of processes operate through the equilibrium adsorption of the gas mixture where the adsorptive selectivity is primarily based upon differential equilibrium uptake of one or more species based on parameters such as pore size of the adsorbent. Kinetically based separation involves differences in the diffusion rates of different components of the gas mixture and allows different species to be separated regardless of similar equilibrium adsorption parameters.
Kinetically based separation processes may be operated as pressure swing adsorption (PSA), temperature swing adsorption (TSA), partial pressure swing or displacement purge adsorption (PPSA) or as hybrid processes comprised of components of several of these processes. These swing adsorption processes can be conducted with rapid cycles, in which case they are referred to as rapid cycle thermal swing adsorption (RCTSA), rapid cycle pressure swing adsorption (RCPSA), and rapid cycle partial pressure swing or displacement purge adsorption (RCPPSA) technologies, with the term “swing adsorption” taken to include all of these processes and combinations of them.
Typically, the zeolite adsorbents used in such gas separation processes either have good kinetic separation selectivity for the contaminant or high capacity for the contaminant, but not both. For example, the DDR zeolite, ZSM-58, which can be used to remove CO2 from a natural gas stream, has a high CO2/CH4 kinetic separation selectivity but a lower CO2 capacity. Thus, ZSM-58 is desirable for selectively separating CO2 from CH4 but is limited with regard to how much CO2 can be adsorbed. Conversely, chabasite (CHA) has high CO2 capacity and poor CO2/CH4 kinetic separation selectivity. Thus, while CHA can adsorb a large amount of CO2, CHA is not as selective for CO2 in the presence of CH4 and will also adsorb CH4.
U.S. Pat. No. 7,435,699 reports a non-homogeneous adsorbent with a core and at least one continuous outer layer in which the core has a volume adsorptive capacity of at least 35% of the volume of the adsorbent and the outer layer has a diffusional selectivity greater than 5.
U.S. Patent Publication No. 2012/0222555 reports a gas separation process using a structured particulate bed of adsorbent coated particles laid down in the bed in an ordered manner to simulate a monolith.
However, there is a need to provide additional adsorbent materials with both improved adsorption capacity and selectivity for a gas contaminant, such as CO2, which can be used in various gas separation processes.